1. Field of The Invention
The present invention relates to systems for controlling the power applied to a load, and in particular to a method to improve the performance of pulse width modulation (PWM) amplification.
2. Description of Related Art
For certain load control applications, it is desirable to have a high degree of precision in terms of linearity and power transfer efficiency. For instance, photolithographic systems require high resolution when in the scanning mode. Therefore, power transfer must be highly linear for controlling the positioning stages in the photolithographic systems, requiring that a discrete change in the position control signal to result in a proportional discrete change in the output signal for the positioning stages. At other times, photolithographic systems operating in a stepping mode require rapid changes in the positioning of the stages, which demand efficient power transfer to generate large acceleration and deceleration forces.
One of the highly effective methods of power delivery control is the use of pulse modulation amplifiers. They are used to supply drive current to inductive loads, such as linear, voice-coil, and DC motors. A pulse modulation amplifier, such as a PWM amplifier, receives an analog waveform and outputs a series of square wave pulses. The square wave pulse has an amplitude and duration such that the integrated energy of the pulses is equivalent to the energy of the sampled input analog waveform multiplied by a gain factor created by the amplifier. The resulting PWM waveform may be filtered to produce an analog waveform replicating the original input waveform multiplied by the gain factor. The frequency of the desired sine wave is called the system frequency, while the frequency at which the switch operates is called the modulation or switching frequency.
a. Prior Art H-bridge PWM
A H-bridge PWM is one of many types of pulse modulation amplifiers. FIG. 1 is an example of a basic prior art H-bridge PWM circuit. It consists of 4 power transistors interconnected to form a bridge, with the load, which in this representation is a servomotor, being positioned in the center of the bridge. Each transistor (110, 120, 130, and 140) has a corresponding xe2x80x9cfree wheelingxe2x80x9d diode 150 connected in parallel, in a reverse current direction, from the emitter to collector. The base voltages of the transistors are controlled by the switching servo amplifiers to turn the transistors 110, 120, 130 and 140 xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d in the appropriate manner so as to cause a drive current, Im, to flow through the motor 170 in the desired direction.
The pairs of transistors are controlled in opposition to restrict losses via the xe2x80x9cfree-wheelingxe2x80x9d diodes. They should not be overlapped in operation; otherwise a short circuit may occur which can result in damage to the circuit. For example, in the first half of the period, transistors 110 and 140 are switched xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d depending on the modulation frequency while transistors 120 and 130 are xe2x80x9coffxe2x80x9d, which result in current Im flowing from Vs+100 through the transistor 110, the motor 170, the transistor 140, to Vsxe2x88x92100, in the direction as shown by the arrows. For the second half of the period, transistors 120 and 130 are switched xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d depending on the modulation frequency while transistors 110 and 140 are switched xe2x80x9coffxe2x80x9d, which result in current flowing through the motor in the opposite direction. This xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d cycle of alternate pairs of transistors is continuously repeated as the servo system controls the acceleration of the motor. The direct unfiltered amplifier output is either near the supply voltage or near zero. Continuously varying filtered output levels are achieved by changing only the duty cycle. As the duty cycle or the modulation frequency is increased, the output square waves become more reflective of the sinusoidal input as shown in FIG. 2. The increase in modulation frequency also results in efficiency being quite constant as output power varies.
b. Design Considerations of PWM
The challenge of designing a pulse width modulator is to get enough dynamic range to deliver the specified output while variables such as output current, input voltage, and temperature fluctuate over wide ranges. If output current remains constant, the average energy into the filter inductor must remain constant. As input voltage rises, the energy delivered to the filter inductor in a given time must be increased. If the input voltage is constant but output current decreases, less energy must be delivered to the filter inductor. The only variable the controller has to work with is the pulse width, which must be increased or decreased depending upon the load requirement. Therefore, PWM switching control is highly critical in determining the waveform output.
Furthermore, the design of the PWM has to take into considerations the following desired parameters: low internal losses to provide high operating efficiency, leading to small size and low cost equipment; high signal-to-noise ratio to provide quality power to the load; high modulation frequency to produce a variable frequency sine wave with small ripple current and minimum harmonics to minimize motor heating; and high surge ratings to protect against overcurrent and overvoltage conditions, thus improving reliability.
The main problem to resolve for all high-power amplifier and oscillator equipment is the removal of excess thermal energy produced in active devices, which can include switching resistance, diode forward drops, copper losses, and core losses. The temperature rise of the PWM circuit must be within the allowable limit as prescribed by the manufacturer of each component. The PWM circuit therefore must be designed to withstand worst-case internal power dissipation for considerable lengths of time in relationship to the thermal time constants of the heat sinking hardware. Consequently, the PWM circuit has to have the necessary heat dissipation device to cool itself under worst-case conditions, which include highest supply voltage, lowest load impedance, maximum ambient temperature, and lowest efficiency output level. In the case of reactive loads, maximum voltage-to-current phase angle or lowest power factor must also be addressed. The available cooling methods to remove the thermal generation include natural convection, forced convection, and conduction. If the excess thermal energy is not removed properly, the temperature rise can create circuit failure and/or reduce power delivery efficiency.
The other problem to be resolved is noise, or interference, which can be defined as undesirable electrical signals that distort or interfere with the original or desired signal. Examples of noise sources include thermal noise due to electron movement within the electrical circuits, electromagnetic interference due to electric and magnetic fluxes, and other transients that are often unpredictable. The main techniques used to reduce noise consist of applying shielding around signal wires, increasing the distance between the noise source and signal, decreasing the length that the desired signal must travel, rounding off or smoothing rough edges to reduce the effects of corona, and proper grounding of the entire system.
The ratio of the signal voltage to the noise voltage determines the strength of the signal in relation to the noise. This is called signal-to-noise ratio (SNR) and is important in assessing how well power is being delivered. The higher the SNR, the better the delivery of desired power. PWM amplification system with low SNR may not be suitable for photolithography motor drives and other high performance applications, which may require noise free power.
Further, conventional PWM amplifier systems do not provide drive current in a linear fashion and typically have poor total harmonic distortion (THD) characteristics. The THD and switching transients, which are associated with very high speed rising and falling edges, can cause noise and generate excessive undershoot and overshoot ringing effects. If these voltage spikes were allowed to exist they could cause high stress and possibly destruction of both amplifier and power supply components. To resolve the ringing effects, amplifier must use fast surge suppression to prevent ringing in the output signals.
The system of the present invention addresses and overcomes the difficulties discussed above by decreasing the Metal Oxide Semiconductor Field Effect Transistor (MOSFET) switching transition time in order to reduce the non-linear effects at high speed switching, compensating for the possible overvoltage conditions, increasing the ability of the MOSFET transistor to withstand thermal and mechanical stresses, and reducing the component size. The result is a hybrid, high performance, high-speed, cost-effective, miniaturized pulse width modulation circuit.
The reduction of the MOSFET switching transition time is done by removing certain MOSFET current limiting resistors and reducing the impedance paths between components. As a result, the rise time and the fall time during the switching periods are reduced, which give the system better response in terms of preciseness power delivery and less non-linear disturbances. To compensate for the possible increase in transient voltage and to protect the MOSFET transistors, an ultra-fast transient protection circuitry is installed between the gate and the source, and between the drain and the source of the MOSFET transistors. The transient protection circuitry operates like a low-pass filter to prevent high voltage from being applied across the MOSFET and damage it.
In addition to the overvoltage effects, high frequency switching can also induce electromagnetic interferences and affect the performance of the circuit. In this invention, in addition to conventional noise suppression strategies, Faraday shielding is used to shield the circuit from electric fields generated by static electricity and attenuate the distortion caused by the electromagnetic emission.
A temperature sensor is also provided to monitor the thermal energy generated inside the circuit; it can be configured to alarm or shutdown the PWM circuit, by removing the triggering voltage to the gate of the switching transistor, when the temperature rises above a certain threshold.
The invention also increases the ability of the PWM circuit to withstand thermal and mechanical stresses by having multiple pieces of interconnected Alumina, bonded together by electrical conductors, as the substrate. This method provides more flexibility than that of the prior art, which has only one solid piece of Alumina as a substrate. By having multiples pieces, the substrate has more room to expand and contract in the area between the pieces, and more angles of rotation and eventual displacement since they can flex in unison and in opposition to each other.
The PWM H-bridge essentially contains two electrically equivalent halves. Geometric symmetry of the circuitry is used, in order to preserve electrical and thermal symmetry within the amplifier. This is important, in order to preserve the linearity, and performance of the PWM. Thermal symmetry ensures the device""s thermally dependent characteristics change in unison. Geometric symmetry ensures the impedance characteristics of the two halves of the circuitry remain identical.
Furthermore, Kovar, rather than the prior art cold rolled steel, is used as the sealant and packaging material, because Kovar has a coefficient of thermal expansion that is closer to those of the substrates Alumina and Beryllium Oxide (BeO). As a result, mechanical stress across the junction is reduced.
The invention uses the costly BeO material only for the substrate in the MOSFET transistors, which generate the most heat, and uses the less costly Alumina substrate for the rest of the circuit. In the prior art, either Alumina or BeO is used for the entire circuit. Using BeO for the substrate is usually reserved for very high performance device at the expense of higher cost. In accordance with this invention, BeO is used only where it is needed, which is at the location where heat is generated the most, at the MOSFET location. This approach reduces the cost of the PWM circuit, but does not compromise the performance of the PWM circuit.
As a result of reducing the non-linear effects at high speed, protecting against over-voltage conditions, increasing the physical strength of the circuit to withstand thermal and mechanical stresses, decreasing the size of the heat sink, and reducing the cost of the circuit without compromising performance, this invention allows for the delivery of linear and efficient power to the load in a reliable and cost effective manner.
In another aspect of the present invention, a stage device is disclosed which deploys a control system that includes the PWM system in accordance with the present invention. In a further aspect of the present invention, a lithography system is disclosed which deploys a stage device that incorporates the stage device in accordance with the present invention. In yet another aspect of the present invention, an object is formed by the lithography system in accordance with the present invention.